Precious metal oxidation catalysts are generally used as a means for removing emissions from the exhaust gas of an internal combustion engine. These oxidation catalysts remove CO, HC, and NOx, simultaneously from engine exhaust gases when the air-fuel ratio of the exhaust gas is at a stoichiometric air-fuel ratio.
Exhaust after-treatment systems have included a second three-way catalyst downstream of the first three-way catalyst which is sometimes referred to as a lean NOx, trap (LNT) . Presently, however, the LNT is only a trapping device for NOx during the lean operation. The LNT must be periodically purged in order to maintain its level of efficiency. To meet the purge requirement, the DISC or lean burn engine must periodically operate at an air-fuel ratio rich of stoichiometry. In addition, the LNT has a very narrow operating temperature range beyond which the LNT trapping capacity and efficiency is greatly reduced.
For a DISC engine operating in a stratified mode, the LNT is typically purged by running the engine rich of stoichiometry for two-three seconds every 50 seconds. The engine control system must manage this purge cycle without causing noticeable torque disturbance to the vehicle. Prior art approaches to this problem involve the use of an electronic throttle, or other electrical supplemental torque devices, for example, a combined alternator-starter or continuously variable transmission (CVT). Such schemes, however, require actuators, in addition to the standard sensor set available on a DISC engine.